Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960

Abstract

Aims. We derive accurate parameters related to the CMD, structure and dynamical state of M52 and NGC3960, whose fields are affected by differential reddening. Previous works estimated their ages in the ranges 35−135 Myr and 0.5−1.0 Gyr, respectively. Methods. J, H and Ks 2MASS photometry with errors <0.2 mag is used to build CMDs, radial density profiles, luminosity and mass functions, and correct for differential reddening. Field-star decontamination is applied to uncover the cluster’s intrinsic CMD morphology, and colour–magnitude filters are used to isolate stars with high probability of being cluster members. Results. The differential-reddening corrected radial density profile of M52 follows King’s law with core and limiting radii of Rcore = 0.91 ± 0.14 pc and Rlim = 8.0 ± 1.0 pc. NGC3960 presents an excess of the stellar density over King’s profile (Rcore = 0.62 ± 0.11 pc and Rlim = 6.0 ± 0.8 pc) at the center. The tidal radii of M52 and NGC 3960 are Rtidal = 13.1 ± 2.2 pc and Rtidal = 10.7 ± 3.7 pc. Cluster ages of M52 and NGC3960 derived with Padova isochrones are constrained to 60 ± 10 Myr and 1.1 ± 0.1 Gyr. In M52 the coreMF (χcore = 0.89 ± 0.12) is flatter than the halo’s (χhalo = 1.65 ± 0.12). In NGC3960 they are χcore = −0.74 ± 0.35 and χhalo = 1.26 ± 0.26. The mass locked up in MS/evolved stars in M52 is ∼1200 Mʘ, and the total mass (extrapolated to 0.08 Mʘ) is ∼3800 Mʘ. The total mass in NGC3960 is ∼1300 Mʘ. Conclusions. Compared to open clusters in different dynamical states studied with similar methods, the core and overall parameters of M52 are consistent with an open cluster more massive than 1000 Mʘ and ∼60 Myr old, with some mass segregation in the inner region. The core of NGC 3960 is in an advanced dynamical state with strong mass segregation in the core/halo region, while the somewhat flat overall MF (χ ≈ 1.07) suggests low-mass star evaporation. The excess stellar density in the core may suggest post-core collapse. The dynamical evolution of NGC3960 may have been accelerated by the tidal Galactic field, since it lies ≈0.5 kpc inside the Solar circle.